1. Field of the Invention
The present invention generally relates to a semiconductor light-emitting device, and more specifically, it relates to a semiconductor light-emitting device so improved that a large quantity of light is extractable. The present invention also relates to a method of manufacturing a transparent conductor film, and more specifically, it relates to a method of manufacturing a transparent conductor film improved to be smooth at a lower temperature and have low resistance and high transmittance so that the cost can be reduced. The present invention further relates to a method of manufacturing a compound semiconductor light-emitting device employing such a method.
2. Description of the Prior Art
FIGS. 9A and 9B illustrate the structure of a conventional light-emitting device and the light-emitting mechanism thereof. Referring to FIGS. 9A and 9B, a light-emitting diode (LED) is an electrical/optical conversion type semiconductor light emitting device utilizing minority carrier injection in a p-n junction part formed by p- and n-type semiconductor crystals adjacent to each other and subsequent recombination radiation.
The device itself is made of a semiconductor crystal material of about 0.3 mm square, and the basic structure thereof is similar to that of an Si rectifying device, as shown in FIG. 9A.
When applying positive and negative forward voltages to the p-type crystal and the n-type crystal respectively, electrons and holes are injected into a p region and an n region respectively, as shown in FIG. 9B. These minority carriers are partially recombined with majority carriers to emit light.
Such an LED has durability, long-livedness, a light weight and a miniature size. The LED, which has been applied only to an indoor indicator lamp, is now applied to the stop lamp of a car, a road sign, a traffic light, a large-area color display or the like following improvement of efficiency and brightness and reduction of the price. Now there is a possibility of applying the LED to the headlamp of a car or a house light substituting for a fluorescent lamp. Further, development of a high-efficiency LED is expected in consideration of energy saving.
Luminous efficiency of an LED includes external quantum efficiency and internal quantum efficiency, and the efficiency of the LED is proportional to the product thereof. The internal quantum efficiency is expressed in the ratio of the number of generated photons to the number of injected electron-hole pairs. In order to improve the internal quantum efficiency, it is necessary to obtain a high-quality crystal containing a small number of defects or impurities, in order to prevent recombination of the electron-hole pairs.
The external quantum efficiency is expressed in the ratio of the number of photons radiated outward to the number of the injected electron-hole pairs. Light generated from an active layer is absorbed by the active layer itself, a substrate or an electrode, and hence can be only partially extracted into air. Further, the refractive index of the semiconductor is by far higher than that of the outside and hence most part of the light is totally reflected by the boundary between the semiconductor and the outside and returned into the semiconductor. Most of LEDs now on the market are sealed with epoxy resin having a refractive index of 1.5, in order to increase the critical angle of total reflection for extracting a larger quantity of light, in addition to the purposes of protection and anti-oxidation of the LEDs.
FIG. 10 is a conceptual diagram showing the structure of a conventional LED. An active layer 23 is formed on an n-type semiconductor layer 22 having an n electrode 21 on the back surface. A p-type semiconductor layer 24 is formed on the active layer 23. A p electrode 25 is formed on the p-type semiconductor layer 24. Recombination radiation occurs at the maximum immediately under an electrode in which a current flows at the maximum. However, a general electrode blocks light, and hence it follows that light emitted immediately under the electrode is hardly extracted outward. In this case, it is important to spread the current to a region other than the electrode. Therefore, a current diffusion layer is provided or a thin gold electrode transmitting light is provided on the overall surface.
FIG. 11 is a sectional view of an LED having a current diffusion electrode 26 provided on a p-type semiconductor layer 24. The current diffusion electrode 26 is formed by an Au thin film of about 20 nm in thickness, in order to obtain sufficient current spreading.
However, the transmittance of the Au thin film 26 having this thickness is only 37% for light of 500 nm in wavelength. Thus, most part of the light is absorbed, to result in inferior luminous efficiency.
Problems of a transparent conductor film applied to a conventional compound semiconductor light-emitting device are now described. The transparent conductor film applied to the conventional compound semiconductor light-emitting device is generally prepared from ITO (In2O3—5 wt. % SnO2). Sputtering is mainly employed for forming a film of ITO having transmittance of at least 80% and resistivity of about 2×10−4 Ωcm at a substrate temperature of 300° C. with excellent reproducibility. In consideration of application to organic electroluminescence (EL) or a light-emitting diode (LED), a transparent conductor film formable at a lower temperature is demanded.
Japanese Patent Laying-Open No. 6-318406 (1994) proposes a technique of manufacturing a film of In2O3—10 wt. % ZnO capable of implementing high transmittance and low resistivity at the room temperature. According to this technique, a film of 140 nm in thickness formed by sputtering under the room temperature implements resistivity of 3×10−4 Ωcm and transmittance of 86% (at 550 nm).
Further, study is made on formation of a transparent conductor film by vapor deposition or ion plating.
The transmittance and conductivity of a transparent conductor film remarkably depend on the amount of oxygen. In the conventional vapor deposition, however, the film forming pressure is disadvantageously limited due to running of a vapor deposition source. In the sputtering, the pressure and the gas for forming a film are limited such that the pressure range is limited for utilizing plasma and argon is required for forming the plasma, and hence the amount of oxygen cannot be precisely controlled.